Democratic presidential candidate Bernie Sanders arrives with staff at a rally in Fairfield, Iowa on January 28, four days before the Iowa caucuses.

Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event at Luther College in Decorah, Iowa on January 24. An estimated 2,000 people attended the rally, a quarter of the town's total population.

A woman grabs Democratic presidential candidate Bernie Sanders for a selfie as he and his wife Jane, center, leave a campaign event at a local campaign office in Muscatine, Iowa on January 29.

Lonnie McAllister and her grandson Vincent McAllister, 9, listen as Democratic presidential candidate Bernie Sanders speaks at a town hall in Independence, Iowa on January 24.

Democratic U.S. presidential candidate Bernie Sanders looks over his speech with Mayor Paul Soglin and Sanders' personal aide Shannon Jackson backstage before a campaign rally in Madison, Wisconsin on April 3.

U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event Grinnell College in Grinnell, Iowa on January 25.

Democratic presidential candidate Bernie Sanders takes the stage as his wife Jane waves to the crowd at a campaign rally and concert at the University of Iowa in Iowa City, Iowa on January 30. Almost 4,000 attended the rally, where bands Foster the People and Vampire Weekend opened, but Bernie Sanders drew the most applause.

People photograph as Democratic presidential candidate Bernie Sanders takes the stage at an African-American Community Conversation town hall event in Milwaukee, Wisconsin on April 2. Sanders struggled behind opponent Hillary Clinton to garner the support of the black community, losing most southern states.

A couple laughs as a man holds a mascot head in the likeness of Democratic presidential candidate Bernie Sanders while waiting in the overflow area before a rally at Temple University in Philadelphia, Pennsylvania on April 6.

A young woman, who spent much of the rally crying with joy, gazes as she holds the hand of Democratic presidential candidate Bernie Sanders after a rally at Temple University in Philadelphia, Pennsylvania on April 6.

Shannon Jackson, aide to Democratic presidential candidate Bernie Sanders, speaks to Sanders and the lead Secret Service agent as they head through a crowd to speak to a Communication Workers of America protest in Philadelphia, Pennsylvania on April 6.

Democratic presidential candidate Bernie Sanders takes the stage with is wife Jane at a campaign rally in Madison, Wisconsin on April 3.

Theodore Thomas Jr., organist at Tindley Temple United Methodist Church, does arrangements as Democratic presidential candidate Bernie Sanders speaks at an African American Community Conversation town hall event in Philadelphia, Pennsylvania on April 6.

Supporters of Democratic presidential candidate Bernie Sanders listens as he speaks at the University of Wisconsin-Eau Claire in Eau Claire, Wisconsin on April 2.

Democratic presidential candidate Bernie Sanders speaks to the overflow crowd at a campaign rally in Milwaukee, Wisconsin on March 29, 2016.

U.S. Democratic presidential candidate Bernie Sanders greets supporters after speaking during a campaign event at Grinnell College in Grinnell, Iowa on January 25.

Democratic presidential candidate Bernie Sanders speaks to the media after what was billed as a private event for his staff, volunteers, and supporters at his campaign's Iowa headquarters in Des Moines, Iowa on February 1.

Democratic presidential candidate Bernie Sanders leaves his campaign charter plane and heads to his motorcade en route to a campaign rally at the University of Wyoming in Laramie, Wyoming on April 5. Sanders was the only Democratic candidate to travel to Wyoming and would win their caucuses a few days later.

Democratic presidential candidate Bernie Sanders arrives with staff at a rally in Fairfield, Iowa on January 28, four days before the Iowa caucuses.

Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event at Luther College in Decorah, Iowa on January 24. An estimated 2,000 people attended the rally, a quarter of the town's total population.

A woman grabs Democratic presidential candidate Bernie Sanders for a selfie as he and his wife Jane, center, leave a campaign event at a local campaign office in Muscatine, Iowa on January 29.

Lonnie McAllister and her grandson Vincent McAllister, 9, listen as Democratic presidential candidate Bernie Sanders speaks at a town hall in Independence, Iowa on January 24.

Democratic U.S. presidential candidate Bernie Sanders looks over his speech with Mayor Paul Soglin and Sanders' personal aide Shannon Jackson backstage before a campaign rally in Madison, Wisconsin on April 3.

U.S. Democratic presidential candidate Bernie Sanders speaks at a town hall campaign event Grinnell College in Grinnell, Iowa on January 25.

Democratic presidential candidate Bernie Sanders takes the stage as his wife Jane waves to the crowd at a campaign rally and concert at the University of Iowa in Iowa City, Iowa on January 30. Almost 4,000 attended the rally, where bands Foster the People and Vampire Weekend opened, but Bernie Sanders drew the most applause.

People photograph as Democratic presidential candidate Bernie Sanders takes the stage at an African-American Community Conversation town hall event in Milwaukee, Wisconsin on April 2. Sanders struggled behind opponent Hillary Clinton to garner the support of the black community, losing most southern states.

A couple laughs as a man holds a mascot head in the likeness of Democratic presidential candidate Bernie Sanders while waiting in the overflow area before a rally at Temple University in Philadelphia, Pennsylvania on April 6.

A young woman, who spent much of the rally crying with joy, gazes as she holds the hand of Democratic presidential candidate Bernie Sanders after a rally at Temple University in Philadelphia, Pennsylvania on April 6.

Shannon Jackson, aide to Democratic presidential candidate Bernie Sanders, speaks to Sanders and the lead Secret Service agent as they head through a crowd to speak to a Communication Workers of America protest in Philadelphia, Pennsylvania on April 6.

Democratic presidential candidate Bernie Sanders takes the stage with is wife Jane at a campaign rally in Madison, Wisconsin on April 3.

Theodore Thomas Jr., organist at Tindley Temple United Methodist Church, does arrangements as Democratic presidential candidate Bernie Sanders speaks at an African American Community Conversation town hall event in Philadelphia, Pennsylvania on April 6.

Supporters of Democratic presidential candidate Bernie Sanders listens as he speaks at the University of Wisconsin-Eau Claire in Eau Claire, Wisconsin on April 2.

Democratic presidential candidate Bernie Sanders speaks to the overflow crowd at a campaign rally in Milwaukee, Wisconsin on March 29, 2016.

U.S. Democratic presidential candidate Bernie Sanders greets supporters after speaking during a campaign event at Grinnell College in Grinnell, Iowa on January 25.

Democratic presidential candidate Bernie Sanders speaks to the media after what was billed as a private event for his staff, volunteers, and supporters at his campaign's Iowa headquarters in Des Moines, Iowa on February 1.

Democratic presidential candidate Bernie Sanders leaves his campaign charter plane and heads to his motorcade en route to a campaign rally at the University of Wyoming in Laramie, Wyoming on April 5. Sanders was the only Democratic candidate to travel to Wyoming and would win their caucuses a few days later.